1. Field of the Invention
The present invention relates to a rubber composition and, more particularly, to a rubber composition comprising a diene-type rubber as a principal rubber component, the diene-type rubber comprising a specific amount of a specific styrene-butadiene copolymer which has been previously extended using an extender oil. The rubber composition exhibits excellent low rolling resistance, high wet skid properties, and superior wear resistance so that the composition can be suitably used as a vulcanized rubber for tire tread, in particular.
2. Description of the Background
Development of a rubber material exhibiting reduced rolling resistance for use in tires is desired to respond to a recent demand for vehicles with a low fuel consumption. In order to reduce the rolling resistance of tires, an energy loss of vulcanized rubber at a low frequency should be decreased. Tires have a temperature of 50.degree. C.-70.degree. C. during operation. A frequency of the external force applied to the tires (or tread rubber), determined by the speed of the vehicle and the diameter of tires, is several tens of Hz. The smaller the energy loss under the low frequency conditions at the above temperature, the lower the fuel consumption of the tires. The concept of "an energy loss of vulcanized rubber under low frequency conditions" can thus be utilized as an index for evaluating the vulcanized rubber in relation to the fuel consumption under practical service conditions of tires. On the other hand, "tan .beta. at 60.degree. C." (wherein "tan .delta." indicates an energy loss) is used as another index (laboratory index) for evaluating vulcanized rubber, which relates to the fuel consumption when a run is reproduced in a laboratory without causing a vehicle to actually run. The smaller the "tan .delta. at 60.degree. C.", the lower the fuel consumption of the tires.
Another requirement for vehicles is the improvement of traveling stability. To respond to this requirement, a rubber material for tires which can increase the frictional resistance of tires on a wet road surface (wet grip) and the frictional resistance on a dry road surface (dry grip) is strongly desired. To increase the frictional resistance of tires on a road surface (particularly on a wet road surface) an energy loss of vulcanized rubber at high frequency should be increased. When a vehicle brakes, the temperature of the tires increases to 50.degree. C.-70.degree. C. In this instance, the tires (or tread rubber) receives an external force at a high frequency (on the order of several tens of thousands to several hundred thousands Hz) due to the invisible irregularity of the road surface. The larger the energy loss at this temperature under the high frequency conditions, the larger the friction resistance of the tires on the road surface, i.e. the tires have a superior grip power. Thus, the concept of "an energy loss of vulcanized rubber under high frequency conditions" can be utilized as an index for evaluation of the vulcanized rubber relating to the grip power in the practical service conditions of tires. However, because it is difficult to obtain a testing machine to measure the energy loss under such high frequency conditions, this cannot be used as a laboratory index of vulcanized rubber relating to grip power as is. Because of this, the frequency is converted into a temperature (under the conditions that the frequency is decreased and the temperature is lowered according to decrease in the frequency) to measure "tan .delta. at 0.degree. C.", which is regarded as the laboratory index of vulcanized rubber relating to grip power. The larger the value of "tan .delta. at 0.degree. C.", the better the performance of the tires in terms of grip power.
As can be understood from the above discussion, there is an antinomic relationship between low rolling resistance (a decrease in fuel consumption) and high friction resistance on a wet surface (an increase in traveling stability). It has been difficult to make these properties compatible. A number of rubber materials which have both of the properties have been proposed. Examples of materials that have been proposed heretofore include: a copolymer, obtained by the copolymerization of a conjugation diolefin and an aromatic vinyl compound using a lithium amide initiator (Japanese Patent Application Laid-open No. 279,515/1994), a styrene-butadiene copolymer, obtained by modifying or coupling the terminals of a polymer obtained using an organic lithium initiator (Japanese Patent Application Laid-open No. 22,940/1989), a rubber composition, comprising a styrene-butadiene rubber and a vinyl polybutadiene (Japanese Patent Application Laid-open No. 183,868/1997), and the like.
However, these rubber materials can not sufficiently satisfy the recently required values for both the above-mentioned properties.
Furthermore, the light weight for tires as well as the above-mentioned low rolling resistance are strongly demanded for the reduction of fuel consumption (fuel consumption improvement). Such a demand has encouraged development of vulcanized rubbers with superior wear properties. In order to decrease the fuel consumption, the energy loss (tan .delta. at 60.degree. C.) of the rubber material itself used as tread must be decreased as mentioned above, and the tires must have a small thickness to reduce weight. Among these, decreasing the weight of tires (decreasing the thickness of tires) inevitably reduces the thickness of the tread. Because of this, development of vulcanized rubber with improved wear properties which can avoid a decrease in the life of tires is desired.
However, as mentioned above it is difficult to produce a rubber material for tires having both superior low rolling resistance and high friction resistance on a wet surface (high wet skid properties). It is even more difficult to provide a rubber material exhibiting improved wear resistance in addition to these properties.
The present invention has been completed in view of these problems and has an object of providing a rubber composition exhibiting excellent low rolling resistance, high wet skid properties, and superior wear resistance, which is suitable for use as a vulcanized rubber, particularly for high performance tires and low fuel consumption tire tread.